This invention relates to satellite antennas and, more particularly, to a low cost mounting and tracking assembly for such an antenna.
The Department of Defense is presently developing a global broadcast service (GBS) which uses satellites. In the interest of cost savings, these satellites are not stationary relative to the earth. Instead, the satellites wander within an approximately .+-.10.degree. range in inclined orbits. The receiving stations for the GBS are either transportable or fixed, although when in use they remain in fixed positions. Therefore, whenever a receiving station is set up, the antenna must be pointed at a GBS satellite in view. Since the satellite wanders, the antenna must be movable to track the satellite after it is initially acquired. Further, the satellite mounting and positioning assembly must be rigid because the antenna is exposed (i.e., not within a radome) and is subject to winds.
Mounting and tracking assemblies for satellite antennas have been in use for many years. The most common type of such assembly is the elevation over azimuth two-axis gimbaled servo system. In such a system, the antenna is attached to an inner elevation gimbal which is supported through preloaded bearings on an outer azimuth gimbal structure. A drive motor, either rotary or linear, provides relative motion between the elevation and azimuth gimbals. The azimuth gimbal, in turn, is supported by preloaded bearings on a fixed structure and another drive motor provides relative motion between the azimuth gimbal and the fixed structure. The realities of physical packaging and the need to nest the elevation assembly inside the azimuth assembly when using such a system tend to make the use of linear, high gear ratio, actuators difficult, and push designers to rely upon rotary, low gear ratio, motors. The parts count, size, inertia and cost are comparatively high. Standardized gimbals do not exist, and therefore each application requires the design and fabrication of many large, customized, mechanical parts and precision features for mounting motors and bearings. Sealing against the environment also becomes an issue with such a system. As such, it provides the capability of directing the antenna line-of-sight in any direction in azimuth, and typically from 0.degree. (horizontal) to +90.degree. (vertical) in elevation. Accordingly, this type of positioner provides full hemispheric coverage. It is also known that elevation over azimuth positioners using linear actuators provide limited hemispheric coverage.
When applied to the problem of tracking an inclined satellite, as in the GBS, the gimbaled system previously described has a number of shortcomings. For example, full upper hemispherical coverage is not needed. When tracking a satellite in, for example, a 10.degree. inclined orbit, the overall field-of-regard must be at least .+-.10.degree. in both elevation and azimuth. The added rotational capability afforded by the gimbals results in unnecessary complexity, lower reliability, and increased cost. Further, the use of rotary motors and gearheads permits the back driving of the antenna in response to wind loading. The gear ratio in this type of system must be relatively low in order to provide the capability to slew the line-of-sight at high speed (i.e., 10.degree. to 60.degree. PER second). This is necessary when the system is scanning for the satellite during initial acquisition. With these low gear ratios, it is possible for external disturbance torques to induce motion that drives the antenna line-of-sight off the satellite. Still further, most systems of this type use brushless DC motors or stepper motors. A disadvantage of this approach is that both types of motors require a motor drive amplifier.
Another type of assembly developed to track moving satellites involves the use of a fixed antenna with a movable feed which can effect a change in line-of-sight direction. The use of a fixed antenna permits the antenna to be rigidly attached to a base structure, which permits operation in high wind and eliminates the need for any type of antenna motion actuator. However, the range of motion possible using this approach is limited to about two beamwidths, which is often insufficient. For example, with a typical 0.75 meter 20 GHz dish antenna the beamwidth is 0.8.degree.. Thus, a width of two beamwidths cannot track a satellite with an inclination greater than approximately 1.6.degree..
For all of the foregoing reasons, it would be desirable to provide a low cost satellite antenna tracking assembly which overcomes all of the foregoing problems.